Fiber-optic sensors are increasingly being used as devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also displacements, vibrations, pressure, acceleration, rotations, or concentrations of chemical species. The general principle of such devices is that light from a laser is sent through an optical fiber and there experiences subtle changes of its parameters either in the fiber or in one or several fiber Bragg gratings and then reaches a detector arrangement which measures these changes.
The growing interest in fiber optic sensors is due to a number of inherent advantages:                Inherently safer operation (no electrical sparks)        Immunity from EMI (electromagnetic interference)        Chemical passivity (not subject to corrosion)        Wide operating temperature range (wider than most electronic devices)        Electrically insulating (can be used in high voltage environment)        
In particular a growing application field is the use of fiber optic sensing system for seismic sensing when deployed in monitoring or production wells. Such sensors are particularly attractive because more conventional seismic sensing systems are expensive to build, and in liquid environments susceptible to failure making them more expensive and difficult to maintain, Additionally, electrical seismic systems are not well suited for in-well installation due to the hostile environment (pressures, temperatures, corrosion). Fiber optic systems do not suffer many of the limitations of electronics and are thus emerging as the technology of choice. It is known that fiber optic seismic sensing cables with single point sensors have been deployed in wells.
From U.S. Pat. No. 5,285,424 it is known that air-backed hydrophones or hydrophones with compliant backing can provide greater sensitivity than hydrophones with solid backing materials. Other prior art (U.S. Pat. No. 7,840,105 B2, U.S. Pat. No. 6,211,964 B1) has shown that acoustic sensors can be built into towed arrays for seismic sensing. These solid cables though would be difficult to couple to a down hole formation—a requirement for good seismic sensitivity.
A fiber optic sensor system may include sensitive fiber optic sensors such as accelerometers, geophones, and hydrophones, which are based on interferometric principles. The method and apparatus to be described herein is completely compatible with Distributed Acoustic Sensing (DAS) systems and other interferometric sensing system used in seismic monitoring. A key design need, particularly for well formation seismic work, is for the interferometric sensing system to be closely coupled to the formation being measured while maintaining high seismic sensitivity by the presence of an inner hollow core. This close coupling for maintaining high seismic sensitivity is referred to as seismic coupling.